The ultimate goal of this project is to use mouse models to better understand the genetic basis of human Down syndrome. The primary aim is to identify the genes on human chromosome 21 which, when present in three copies, result in the congenital heart disease and Alzheimer-like neuropathological changes characteristic of Down syndrome individuals. Regions of genetic homology between human chromosome 21 and mouse chromosomes 16, 17, and 10 have been identified. Trisomy 16 mice demonstrate congenital heart disease and thymic and hematopoietic developmental abnormalities similar to those seen in human Down syndrome. Trisomy 10 mice also exhibit congenital heart disease like that seen in Down syndrome. It is not know if these features are due to trisomy for genes shared in common with human chromosome 21 or not. The Alzheimer- like changes seen in the brains of adult Down syndrome patients have not been amenable to testing in a mouse model because the trisomic mice die as fetuses. In order to determine if the congenital heart disease of trisomy 16 and trisomy 10 mice is due to reduplication of genes homologous to those on human chromosome 21, just the homologous regions of mouse chromosomes 16 and 10 could be inserted into mice in three copies. Individuals possessing these extra defined small segments could be examined to see which, if any, of the segments, when present in three copies, produce the phenotype. Insertion of small regions should also allow viability and thus testing for Alzheimer-like brain changes. To accomplish this, the extent of the regions of mouse chromosomes 16 and 10 homologous to human chromosome 21 will be defined using physical mapping techniques in somatic cell hybrid lines. The homologous regions will then be cloned as large pieces in yeast artificial chromosomes, and these large pieces will be inserted into pluripotent mouse teratocarcinoma cell lines using protoplast fusion techniques. These transformed cell lines, bearing small defined segments of mouse chromosomes in three copies, can then be tested for expression of the inserted sequences and used to generate mice as genetic models of Down syndrome.